Capacitance electronic discs are a recently developed form of molded information storage media.
It was disclosed in U.S. Pat. No. 4,044,379 issued to Jerome Bart Halter entitled, "METHOD AND APPARATUS FOR ELECTROMECHANICAL RECORDING OF SHORT WAVELENGTH MODULATION IN A METAL MASTER", that satisfactory recordings for capacitance electronic discs can be made by electromechanically cutting the recording into a metal substrate. More particularly, it was disclosed in the Halter patent that the preferred substrate for recording of information for capacitance electronic discs is electrodeposited copper on an aluminum support.
In order to obtain the optimum properties in the recorded disc, it has been found that the copper should be electrodeposited in a bright state. Electrodeposited bright copper is distinguishable from conventional electrodeposited copper in that bright copper as deposited forms a bright, lustrous, almost mirror-like layer which is further characterized as being smoother, harder, and having an extremely small grain size as compared to conventional electrodeposited copper.
Bright electrodeposited copper is obtained by adding certain additives to conventional acid copper deposition baths which cause the copper to be deposited in the bright state. Various additives have been suggested in the prior art such as in U.S. Pat. No. 2,424,887 issued to Henricks entitled, "METHOD AND ELECTROLYTE FOR ELECTRODEPOSITION OF METAL"; U.S. Pat. No. 2,707,166 issued to Brown entitled, "ELECTRODEPOSITION OF COPPER FROM ACID BATH", and U.S. Pat. No. 3,267,010 issued to Creutz et al. entitled, "ELECTRODEPOSITION OF COPPER FROM ACIDIC BATHS".
There are likewise a variety of commercial products sold as brighteners for use in the electroplating of copper. One such product which is widely used is known as UBAC #1 and is sold by the Oxy Metal Industries Corporation, Detroit, Michigan.
The brighteners are very effective in promoting the deposition of the copper in bright lustrous layer. The resulting bright copper plating obtained with the brightener additives, particularly those disclosed in U.S. Pat. No. 3,267,010 referred to above and with commercial products such as UBAC #1, have been found to be satisfactory for most applications.
It has been found, however, that electrodeposited bright copper deposited from the bright acid copper bath disclosed in the prior art is not completely satisfactory for recording substrates for capacitance electronic discs. It has been found that using the bath composition and methods of deposition disclosed in the prior art, that the deposited copper often has internal voids which interfere with the recording of the substrates.
The plating baths which are employed for the electrodeposition of bright copper have a somewhat less than 100% current efficiency with regard to plating of the copper from the bath. Typically the baths have an efficiency of about 96% to 98%. The inherent inefficiencies in the utilization of the current for plating results in hydrogen gas being liberated at the cathode. While some of the hydrogen escapes to the atmosphere, it has been found that a portion of the hydrogen becomes occluded in the deposited copper and causes the formation of small voids in the plated layer of copper. The voids formed by the hydrogen gas bubbles are relatively small, generally being at most a few microns in size. The occluded hydrogen and the resulting voids in the deposited layer of copper are, however, especially troublesome in the substrates used for the recording of the information for the capacitive electronic discs. The voids can result in major defects in the final molded capacitance electronic disc. Because of the small size of the signal elements and the closeness of the signal tracks on the surface of the capacitance electronic disc, voids of only a few microns in width can extend over the width of several signal tracks causing a distortion of the recorded signal element. A further problem is that the voids can often result in tracking problems of the playback stylus causing the stylus to lock on a given signal track or, in the alternative, to skip signal tracks as a result of the stylus being caught in the voids.
In order to reduce the effect of the generated hydrogen on the final deposited copper metal, attempts have been made to decrease the surface tension of the plating bath by adding various types of surfactants to the plating bath, as suggested in the prior art for other plating baths such as semi-bright nickel baths. These techniques did not prove to be successful. Most of the surfactants which were evaluated were hydrocarbon based materials. The hydrocarbon based surfactants had the distinct disadvantage that when they were introduced into the plating bath, they were found to be incompatible with the acid plating baths and would either settle out or breakdown in the plating bath. Those hydrocarbon based surfactants which did not break down or did not separate from the plating bath had the further and possibly more serious deficiency that when added to the bright acid copper bath, the surfactant interfered with and in many cases completely destroyed the effectiveness of the brightener additive, and as a result the copper deposited in the form of a dull, flat, rough layer. As a result of the problems encountered in adding surfactants to bright copper baths, manufacturers of brightener compositions, as for example the manufacturer of UBAC #1 mentioned above, advise platers in their technical literature that surfactants should not be added to plating baths which contain brighteners.
It would be highly advantageous in the manufacture of bright copper recording substrates for capacitance electronic discs if a method could be provided to prevent the occlusion of hydrogen bubbles in the deposited copper layers and the resulting formation of voids without adversely affecting the brightness and other properties of the deposited copper.